The energy-efficient Hemi process is becoming the preferred method for making phosphoric acid worldwide. This paper compares the Hemi process with the earlier Dihydrate process in terms of capital and operating costs, energy efficiency, and consumption of raw materials.

Phosphoric acid is the predominant raw material for production of most phosphate fertilizers. The Hemi process reacts phosphate concentrate with sulfuric acid to produce phosphoric acid at substantially higher concentration than the traditional Dihydrate process. This more concentrated acid avoids need for most of the processing and energy required for further concentration.

Heat for concentration of phosphoric acid is supplied as steam from process heat from adjacent sulfuric acid plants. With the Hemi process, less of the steam from sulfuric acid processing is required to concentrate phosphoric acid, so more steam can be utilized to generate electric power. A typical Hemi phosphoric acid plant with capacity for 1500 tons P2O5/day allows about 150 MW more electric power production from this steam. Capital cost of the entire facility is typically less than with a Dihydrate process. Less equipment is required for concentrating and handling phosphoric acid, and for cooling water associated with concentration. The Hemi process can handle unground phosphate concentrate, whereas the Dihydrate normally requires an expensive grinding operation. Reaction, filtration, and fume scrubbing facilities are normally more expensive with the Hemi process. The Hemi process saves about 2% in sulfuric acid consumption. There is usually little difference in phosphate recovery throughout the facility. A variant called Hemi-Di provides exceptionally high phosphate recovery, but requires substantially more capital investment. Product acid is purer, which can be a substantial benefit where higher fertilizer analysis is desired.

Many Hemi phosphoric acid plants have been built worldwide. Their track record is covered, including capacities, dates, and source of phosphate. Conversions of some Dihydrate plants to the Hemi process are described.


This article explains why the energy-efficient Hemi (hemihydrate) has become the preferred process for making phosphoric acid in the 21st century. The related Hemi-Di process begins like the Hemi process and adds process steps for extra benefits. The Dihydrate (Di) process served as the standard of the industry for several decades. Other phosphoric acid processes including Di-Hemi and a short-cut Hemi-Di have also found their niche.

The term hemi refers to calcium sulfate hemihydrate. A phosphoric acid plant produces far more gypsum (calcium sulfate) than phosphoric acid. A dihydrate process makes the dihydrate form of gypsum, and a hemi plant makes hemihydrate gypsum - involving higher combination of temperature and concentration. During the last few decades people have developed ways to enjoy hemi=s high concentration advantage without suffering its potential chaos.

This content is only available via PDF.
You can access this article if you purchase or spend a download.